feat: add GitHub Discussions as a comment provider via Giscus (#2830)

modules/wowchemy/layouts/partials/comments/giscus.html

@@ -0,0 +1,19 @@
+{{ if site.Params.features.comment.giscus }}
+  {{ $config := site.Params.features.comment.giscus }}
+  <script src="https://giscus.app/client.js"
+          data-repo="{{$config.repo}}"
+          data-repo-id="{{$config.repo_id}}"
+          data-category="{{$config.category}}"
+          data-category-id="{{$config.category_id}}"
+          data-mapping="{{ $config.mapping | default "pathname" }}"
+          data-strict="0"
+          data-reactions-enabled="1"
+          data-emit-metadata="0"
+          data-input-position="top"
+          data-theme="{{ $config.theme | default "preferred_color_scheme" }}"
+          data-lang="{{ $config.language_code | default "en" }}"
+          data-loading="lazy"
+          crossorigin="anonymous"
+          async>
+  </script>
+{{ end }}